As filter medium for an air filter to meet the criteria for high efficiency particulate air (HEPA) filters, filter medium fabricated using glass fibers, called glass filter medium, are known. While glass filtration media of HEPA grade have a high collection efficiency; i.e., 99.97%, for particles having a particle diameter of 0.3 μm, pressure drop is high.
As HEPA filters with low pressure drop in place of this kind of glass filter medium, filter medium using PTFE porous film produced by stretching polytetrafluoroethylene (PTFE) are known. Compared to glass filter medium, PTFE porous films have higher collection efficiency and lower pressure drop, and have an excellent balance between collection efficiency and pressure drop.
For example, in Japanese Laid-open Patent Publication Nos. 2013-52320 and 2013-63424 noted hereafter, to suppress pressure drop for an air filter unit used while retained by a shape retaining member, there are proposed filter medium for an air filter for which a melt-blown nonwoven is provided on the upstream side of the PTFE porous film, and an air-permeable cover layer composed of a spun-bonded nonwoven is further provided to the upstream side.